Nitric oxide has recently been found to play an important role in life processes in humans and animals. For example, it helps maintain blood pressure by dilating blood vessels, and kills foreign invaders in the body's immune system. Studies indicate that extraordinary benefits may be obtained by administering small dosages of nitric oxide to patients who suffer from certain illnesses or diseases. Of particular interest is the prospect of reducing pulmonary vasoconstriction in pediatric patients with congenital heart disease complicated by pulmonary artery hypertension by having the patients inhale oxygen-enriched air containing very small concentrations of nitric oxide.
Nitric oxide is a relatively stable gas when it is in the pure state or mixed with an inert gas, such as nitrogen or argon. However when it is mixed with oxygen it reacts rapidly with the oxygen to form nitrogen dioxide, a substance that is highly toxic to humans. The nitrogen dioxide reacts with water to form nitric and nitrous acids, which, when inhaled can cause severe pulmonary oedema, acid pneumonitis or even death. Because of the highly toxic character of nitrogen dioxide, nitric oxide that is intended for inhalation use by humans is generally purified to remove any nitrogen dioxide that is initially in the nitric oxide product as a result of the manufacturing process, and the purified product is stored and shipped in an oxygen-free environment to prevent the subsequent generation of nitrogen dioxide in the storage or shipping container.
Nitric oxide is generally administered to a patient by diluting a nitrogen-nitric oxide concentrate gas containing about 1000 ppm nitric oxide with oxygen or oxygen-enriched air carrier gas to produce an inhalation gas containing nitric oxide in the desired concentration range (usually about 0.5 to 200 ppm, based on the total volume of the inhalation gas). Calculations based on nitric oxide chemical kinetics suggest that if pure oxygen is mixed with the above-described nitrogen-nitric oxide concentrate to produce a gas mixture having a nitric oxide concentration of 200 ppm, it takes only about 3 seconds for the concentration of nitrogen dioxide in the gas mixture to build up to 3 ppm. The currently accepted upper limit for nitrogen dioxide inhalation is 5 ppm (based on the total volume of breathing gas being inhaled). Assuming that this gas mixture is inhaled by a patient within 3 seconds after mixing the nitrogen-nitric oxide concentrate and oxygen, the amount of nitrogen dioxide initially present in the nitric oxide-nitrogen concentrate would have to be very low to ensure that the nitrogen dioxide concentration in the inhalation gas does not exceed 5 ppm. To minimize the risk of exceeding the 5 ppm upper limit, it is desirable that the concentration of nitrogen dioxide in the nitrogen-nitric oxide supply vessel be as low as possible, and it is most preferred that it not exceed about 1 ppm.
Nitrogen dioxide is produced as a byproduct of most, if not all, nitric oxide production processes. Various techniques are employed to remove nitrogen dioxide from the nitric oxide. U.S. Pat. No. 3,489,515 discloses the purification of nitric oxide by washing the nitric oxide with a dilute aqueous solution of nitric acid. The water reacts with the nitrogen dioxide to produce nitric and nitrous acids, which can be washed from the gaseous product stream by washing the stream with water. This method is not satisfactory for producing medical grade nitric oxide because it does not adequately reduce the concentration of nitrogen dioxide in the product gas stream. Nitrogen dioxide can also be removed from nitric oxide by cryogenic distillation. This method likewise leaves a lot to be desired because of the high capital cost of distillation equipment and because not all of the valuable nitric oxide is recovered during the distillation.
Another purification technique that has been reported is adsorption using as adsorbent a bed of the activated coke or activated charcoal (see U.S. Pat. Nos. 2,568,396 and 4,149,858). This procedure suffers from the disadvantages that the activated coke and activated charcoal do not efficiently remove nitrogen dioxide from the gas stream, they adsorb more nitric oxide than is desired and they tend to catalyze the disproportionation of nitric oxide to nitrogen dioxide and nitrogen.
It is known that certain zeolites adsorb nitrogen dioxide from gas streams containing nitric oxide and nitrogen dioxide without also adsorbing the nitric oxide in the gas stream. For example, U.S. Pat. No. 4,153,429, issued to Matthews et al, which relates to the removal of NO.sub.x from gas streams, discloses the use of zeolite Y adsorbent containing 8 to 30 equivalent percent metal cations to remove nitrogen dioxide from a gas mixture. The patentees assert that zeolites do not adsorb nitric oxide, and to eliminate nitric oxide from the gas mixture, stoichiometric quantities of oxygen must be present with respect to the quantity of nitric oxide to be removed from the gas stream. Nitric oxide is oxidized to nitrogen dioxide in the presence of oxygen.
Copending U.S. patent application Ser. No. 129,647, filed Sep. 30, 1993, discloses the removal of nitrogen dioxide from a nitric oxide gas stream by passing the gas stream through a zeolite selected from types A, X and Y zeolites, mordenite, faujasite and chabazite. Although these adsorbents exhibit superior nitrogen dioxide adsorption properties it has been found that they also possess the undesirable attribute of catalyzing the disproportionation of nitric oxide to nitrogen dioxide and nitrogen. It is believed that the metal cations on the zeolite causes the catalytic reaction.
Because of the high toxicity of nitrogen dioxide, highly effective methods for purifying nitric oxide intended for use in medical applications, without concomitantly producing additional nitrogen dioxide, are continuously sought. The present invention provides a simple and efficient method of achieving this objective.